1. Field of the Invention
The present invention relates to a variable-directivity antenna for use in a device that uses radio frequency electromagnetic waves such as microwaves or millimeter waves.
2. Description of the Related Art
A linear antenna such as a whip antenna for a cellphone is usually designed so as to stand up perpendicularly to the ground when the terminal is used with its body made to stand up to make a call. In that case, the linear antenna has directivity that is isotropic within a horizontal plane, which is defined perpendicularly to its linear feed conductor as shown in FIG. 14. As illustrated in FIG. 14, the terminal 1021 that now stands up perpendicularly to the ground has an antenna radiation directivity pattern 1031 that is parallel to the ground (or a horizontal plane), thus obtaining radiation gains in a broad range. As a result, this terminal 1021 can be used conveniently to access the base station 1001.
When a cellphone is used as an information transmitter-receiver, however, the terminal is often laid down on a desk or something parallel to the ground. In that case, the feed conductor of the linear antenna of such a laid terminal 1022 runs almost horizontally and the direction in which radiation gain is obtained gets tilted with respect to the horizontal plane. As a result, no radiation gains could be obtained in the direction leading to the base station 1001 and the communication sensitivity might decline.
To overcome such a problem, a radiation directivity pattern 1003 in which the radiation directivity of the antenna is varied on a plane including the longitudinal direction of the antenna (i.e., a vertical plane) would be needed.
As for a wireless LAN that is used indoors in most cases, however, the radio waves could sometimes be jammed by comings and goings of people or it could be difficult in some places to establish a communication link due to multi-path phasing. This tendency manifests itself particularly when the frequencies for use to keep up communications increase because the diffraction of electromagnetic waves weakens in that case. That is why this would pose a serious problem when communication systems that use higher frequencies are popularized.
One of the methods for overcoming those problems is a technique for increasing the radiation gain of an antenna in a direction in which a communication link can be established by directly receiving an incoming wave but decreasing the gain in a direction from which a disturbance wave is coming, thereby suppressing interference and increasing the communication sensitivity. For that purpose, an antenna that can change its radiation directivities adaptively according to the status of radio wave propagation is needed.
Meanwhile, an antenna that uses a linear conductor such as a monopole antenna or a dipole antenna has a radiation directivity pattern that is symmetrical with respect to its rotation axis (that runs in the longitudinal direction). A lot of people have proposed an antenna that changes its directivities within a horizontal plane by using an antenna of that type and a passive conductor element in combination. Such an antenna is disclosed in Japanese Patent Application Laid-Open Publication No. 2001-024431, for example.
In such an antenna, however, a gap corresponding to a quarter to a half wavelength needs to be provided between a feed element and a passive element, which is provided outside of the feed element, to optimize the degree of coupling between these two elements. Consequently, the overall antenna will often occupy much space.
A technique for overcoming such a problem is disclosed in Japanese Patent Application Laid-Open Publication No. 2001-127540 (Hereinafter, Patent Document No. 2), for example. Such a technique will be described with reference to FIG. 16. As shown in FIG. 16, the antenna disclosed in that patent document includes not only a linear feed conductor element 163 but also at least two more linear conductors 164 and 166 with mutually different lengths, which are arranged so as to draw a circle around the feed conductor element 163. And those additional linear conductors 164 and 166 are connected together by way of switching elements 165. The antenna further includes members 169 and 160 that are used to connect the antenna to a control section that drives the switching elements 165. And the control section includes a member for driving and turning ON or OFF an arbitrary one of those switching elements.
According to Patent Document No. 2, if a predetermined length is defined by switching those linear passive conductor elements to make those elements function as a waveguide, then the radiation directivity can be established in the direction in which the waveguide is defined and the radiation characteristic can be controlled within a horizontal plane.
In addition, linear passive conductors that are not used may be switched so as to have lengths that do not affect a predetermined electromagnetic wave. That is why the linear passive conductors can be arranged near the feed conductor element and the space occupied by the antenna around the feed conductor element can be cut down.
Suppose such an antenna is used in a mobile telecommunications terminal such as a cellphone. In that case, even if the user holding the cellphone has changed his or her posture or if the communication status has changed, the gain of the antenna could be increased and the communication sensitivity could be improved by controlling the radiation directivity.
According to the technique disclosed in Patent Document No. 2, however, the radiation directivity can be controlled only within a horizontal plane, not within a vertical plane. To change the radiation directivities within a vertical plane, an array may be formed by arranging a number of feed elements in their longitudinal direction (such an array is called a “collinear array”) and the phases may be controlled between those elements. Such a technique is disclosed in Japanese Patent Application Laid-Open Publication No. 05-160630 (Hereinafter, Patent Document No. 3), for example. Hereinafter, the technique disclosed in Patent Document No. 3 will be described with reference to FIG. 18.
The antenna shown in FIG. 18 has two cylindrical conductors 189 and 180, multiple pairs of half-wave dipole antenna elements 183 and a coaxial feeder line 184. The cylindrical conductors 189 and 180 are arranged concentrically with a dielectric member 181 interposed between them. A number of ring slots 182 are arranged periodically on the outer one 189 of the two cylindrical conductors 189 and 180 at an interval of less than 0.7 wavelength. Each pair of half-wave dipole antenna elements 183 is arranged symmetrically around, and interposes, an associated one of the ring slots 182 and implemented as cylindrical skirts. The coaxial feeder line 184 is arranged so as to extend inside the inner one 180 of the two cylindrical conductors 189 and 180. And the coaxial feeder line 184 has an outer conductor and an inner conductor that are respectively electrically continuous with the outer and inner ones 189 and 180 of the two cylindrical conductors.
By arranging those ring slots 182 periodically, each pair of adjacent antenna elements 183 is supplied with electricity so as to have a predetermined phase difference between them. As a result, beam tilting is realized on the vertical plane.
According to the technique disclosed in Patent Document No. 3, however, the radiation directivity cannot be controlled within a horizontal plane. In addition, since a number of antenna elements are arranged vertically in multiple stages, the antenna assembly becomes longer and longer as the number of such stages increases. For that reason, this technique is not effectively applicable to a mobile telecommunications terminal that should have as small a size as possible.
A technique for overcoming such a problem is disclosed in Japanese Patent No. 3491682 (Hereinafter, Patent Document No. 4), for example. The antenna disclosed in that patent document will be described with reference to FIG. 17. The antenna includes a linear radiator (feed element) 170, at least one linear passive element 173, and a U-passive element 171. The linear passive element(s) 173 is/are arranged parallel to the feed element 170 so as to keep a predetermined distance from the feed element 170. If there is only one linear passive element 173, its length may be half as long as a desired transmission frequency. On the other hand, if there are multiple linear passive elements 173, those elements 173 are connected together with switches 172 interposed between them. The U-passive element 171 is arranged near one end of the linear radiator 170 and has two arm portions that are parallel to each other. When viewed on a plane that intersects at right angles with the plane including the two arm portions, the U-passive element 171 is arranged such that one end of the linear radiator 170 is inserted between the two arm portions through the end of the two arm portions.
According to the technique disclosed in Patent Document No. 4, the linear passive element 173 is divided into a number of portions that should be connected together with the switches 172 interposed between them, thereby changing the locations of the passive elements that need to interact with the feed element on both vertical and horizontal planes.
As a result, the radiation directivities can be changed on the vertical plane, too. It should be noted that the U-passive element 171 is provided just to achieve matching and essentially has nothing to do with the control of radiation directivity, which is the problem to be solved by the present invention.
As disclosed in Patent Document No. 4, the passive element used as a reflector should have a length corresponding to approximately a half wavelength. Likewise, the passive elements that are used to form a waveguide should also have a length substantially corresponding to at least a half wavelength to be arranged near the feed element.
To change radiation directivities within the vertical plane, the center of the passive element that needs to function as either a waveguide or a reflector should be shifted in the longitudinal direction (or major-axis direction) with respect to the center of the feed element. That is to say, the center of the passive element should be shifted perpendicularly to the horizontal plane. An example of such design is shown in FIG. 15.
The length L2 of a straight passive element 20 is approximately equal to the combined length D2 of a pair of feed elements 10. To change the radiation directivities in an elevation angle direction on the vertical plane, the center of the straight passive element 20 should be shifted in the longitudinal direction of the pair of feed elements 10. In FIG. 15, this length is identified by L1.
However, the overall length of the antenna should be increased by the magnitudes of shifts of these two centers. Consequently, the antenna will occupy too much space to be used in a mobile telecommunications terminal that should have as small a size as possible.
In order to overcome the problems described above, the present invention has an object of providing an antenna assembly that can control the radiation directivity of a linear antenna such as a dipole antenna on not only the plane including its feed element (i.e., vertical plane) but also the plane intersecting with the feed element at right angles (i.e., horizontal plane) and that does not have its overall antenna length increased by its passive element in the longitudinal (i.e., major-axis) direction.